Theory of Water Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 355

Figure 7.27 Apparatus for measurement of unsaturated soil coefficient of permeability using
steady-state method (after Klute, 1965a)

top of the specimen is denoted as P 1 , and P 2 is the pressure
applied at the bottom of the specimen. Two tensiometers, T 1
and T 2 , are installed along the height of the specimen for
the measurement of pore-water pressures at two points in the
soil. An air supply maintains a constant pore-air pressure to
the soil specimen. The air pressure is measured using the
manometer, M .
A water supply is applied to the top of the upper porous
plate, P 1 , to develop a constant hydraulic head gradient
across the soil. The water supply provides a constant
hydraulic head, h w 1 , by means of a Mariotte bottle. It is
also possible to use a simple overflow system. Water flows
through the ceramic plate P 1 , the soil specimen, and the
ceramic plate P 2 . The outflow of water is maintained at a
constant hydraulic head h w 2 , below the porous plate. Plate
P 2 controls the outflow elevation. Valves S 1 and S 2 ,are
used to flush air bubbles that may accumulate in the water
compartment adjacent to the porous plate.
The permeability test is started using a low matric suction
value. The matric suction is increased in steps as the perme-
ability is measured. In other words, the test is commenced at a
condition near saturation and proceeds through a drying pro-
cess in accordance with the following procedure. The matric

tensiometers installed along the specimen. The pore-water
pressure head is determined from the elevation of the fluid
interface in the attached manometer relative to the tensiome-
ter elevation. The pressure head is positive when the fluid
interface is above the elevation of the tensiometer and is
negative when the interface is below the tensiometer eleva-
tion. The tensiometer readings in Fig. 7.27 indicate pressure
heads of ( + )h p 3 and ( − )h p 4 from tensiometers T 1 and T 2 ,

respectively.
A constant hydraulic head gradient is then applied to
the specimen as depicted by hydraulic heads h w 1 and h w 2 .
Steady-state flow conditions are achieved when the inflow
and outflow rates are equal and the tensiometer readings
are constant with time. The volume of water, Q , flowing
across the cross-sectional area of the soil, A , in a period of
time t is measured and used to compute the flow rate. The
hydraulic heads h w 3 and h w 4 correspond to tensiometers T 1
and T 2 , which are placed a distance d t apart. By knowing
the distance d t , it is possible to compute the hydraulic head
gradient in the soil. The measured hydraulic head gradient
is used in calculating the coefficient of permeability owing
to uncertainties associated with the hydraulic head changes
across the high-air-entry ceramic disks. The contact planes
between the specimen and the porous plates also produce
uncertainties in predicting heads throughout the specimen
(Klute, 1972).

suction u a − u w is set to a specified value by controlling the

pore-air pressure, as indicated by the height in the manome-
ter fluid, h m . The pore-water pressure is measured using the